A conventional heat pump system comprises a compressor, a condenser, an evaporator, a pressure reduction means normally disposed between the condenser and the evaporator and circulating passage connecting the elements above in which a heat transfer medium is circulated. An operation of such apparatus is usually controlled by detection of the outlet pressure at an outlet of the compressor and the inlet pressure at an inlet of the compressor and, based on the detected values, the respective elements of the system may be controlled by, such as, varying the rotational speed of fans associated with the condenser and/or evaporator or the revolutional speed of the compressor.
In the conventional heat pump system referred to above, it has been necessary to provide means such as a pressure switch or a pressure sensor for detecting the degree of pressure at certain points within the system. In a case where the pressure is to be detected at plural points, provision of plural pressure switches or pressure sensors is naturally required which provision necessitates additional expense, thus making the total capital cost of the system expensive. Further, in a case where employment of pressure switches is effected, the critical pressure value therefor is pre-set in advance so that detection of continuous variations in the pressure is impossible, the switch only being operable at the pre-set value. Accordingly, it has been impossible to effectively control the system in response to continuous variations in the pressure, quite apart from the high cost of the total system.